The possession ability of a ghost (DnD 5e) states that it can possess one humaniod it can see within 5ft.
I interpreted that a ghost could use its possession ability from the etherial plane itself, since the ghost is visible on the material when in the etherial plane and as such can see the creature. Like this it is possible for the ghost to come up to a humanoid without being able to be hit and possess the creature unhindered.
If the ghost gets shunted out I rule that it ends up in the material plane as the ghost has to enter the body of the creature (who is in the material) while possessing.
However, one of my players mentioned that it is an action for a ghost to change from the material to the etherial and vice versa. He then argued that the ghost should enter the material plane first as an action first before having to spend another action on the next turn to attempt to possess a humanoid.
I can’t really find anything specific that tells that one or the other is true. Obviously the former method is more powerful and while I don’t want to be unfair to my players, I think it is also a whole lot more interesting.